Jeffo is talking about excise not sales tax and BobGed's link to Indiana's Excise tax explains quite well how it's figured.
In any event Indiana RV annual taxes are substantially less than they were a few years ago when they were considered personal property with taxes collected by the county. The excise tax is now collected by the BMV and then forwarded it to each county (after the state hangs on to it for a while to earn them selves some extra interest).
Between the state's caps of 1% on home owner, 2% on landlord & farm) and 3% on business of assessed real estate value and the reduced RV tax all of our counties, cities, schools, etc. are having a real problem covering the cost of providing services to the tax payers.
Dave